fib_trie: Address possible NULL pointer dereference in resize

If the inflate call failed it would return NULL.  As a result tp would be
set to NULL and cause use to trigger a NULL pointer dereference in
should_halve if the inflate failed on the first attempt.

In order to prevent this we should decrement max_work before we actually
attempt to inflate as this will force us to exit before attempting to halve
a node we should have inflated.  In order to keep things symmetric between
inflate and halve I went ahead and also moved the decrement of max_work for
the halve case as well so we take care of that before we actually attempt
to halve the tnode.

Fixes: 88bae714 ("fib_trie: Add key vector to root, return parent key_vector in resize")
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NAlexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -830,7 +830,7 @@ static struct key_vector *resize(struct trie *t, struct key_vector *tn)
 	/* Double as long as the resulting node has a number of
 	 * nonempty nodes that are above the threshold.
 	 */
-	while (should_inflate(tp, tn) && max_work) {
+	while (should_inflate(tp, tn) && max_work--) {
 		tp = inflate(t, tn);
 		if (!tp) {
 #ifdef CONFIG_IP_FIB_TRIE_STATS
@@ -839,7 +839,6 @@ static struct key_vector *resize(struct trie *t, struct key_vector *tn)
 			break;
 		}
 
-		max_work--;
 		tn = get_child(tp, cindex);
 	}
 
@@ -850,7 +849,7 @@ static struct key_vector *resize(struct trie *t, struct key_vector *tn)
 	/* Halve as long as the number of empty children in this
 	 * node is above threshold.
 	 */
-	while (should_halve(tp, tn) && max_work) {
+	while (should_halve(tp, tn) && max_work--) {
 		tp = halve(t, tn);
 		if (!tp) {
 #ifdef CONFIG_IP_FIB_TRIE_STATS
@@ -859,7 +858,6 @@ static struct key_vector *resize(struct trie *t, struct key_vector *tn)
 			break;
 		}
 
-		max_work--;
 		tn = get_child(tp, cindex);
 	}